Objective: Investigation on the potential carcinogenicity of radioactive precursors of DNA, RNA and proteins, in relation to their specific pattern and sites of incorporation. Tritium labeled precursors, namely thymidine, uridine or leucine, are currently investigated in newborn, juvenile and adult C57 Black mice of either sex, in relation to time sequence of incorporation, and modulation of tumor incidence throughout the life span. Approach: Mice will be observed throughout entire life span (average remaining life time plus or minus 600 days) for determination of longevity and tumor incidence, in an attempt to relate these endpoints to the various incorporation patterns and intracellular distributional doses. Modulation of tumor incidence spectra in the three lines will be investigated, as well as other possible heritable effects of radiation injury, among which reproductive fitness, litter characteristics, life span, and any phenotypic mutation that might occur.